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The riding is easy because the hard work was done back in the days when this country lane served frantic industry instead of leisure.

The trail's slight grade was carved out of rock for the men who sought and won a bonanza in and around Waihi.

The Public Works Department chipped, ripped and blasted their way along the Karangahake Gorge, building three bridges and a tunnel so freight-laden steam trains could cross from Waikato through a gap in the hills to Waihi.

For more than 70 years, the gorge was a celebrated feature of the East Coast Main Trunk Line.

Riding along, I recalled watching the night goods train go through in the 1960s.

The steam locomotive hissed and writhed round the foot of the black hill; firebox flashes bounced off the face of the crew; the searchlight played on the river and cliffs.

Goods trains stopped using the gorge section in 1978 when it was made redundant by the shorter Kaimai Tunnel route between Hamilton and Tauranga.

However, in the gloom of losing a hard-won asset, some people saw a nugget twinkling in the grey gravel track ballast.

The prospect of tourism.

They pushed to keep the route as a public treasure - at a time when the concept of recreational walkways was new.

The NZ Walkway Commission approved the idea of the section between Waikino Station and Waihi going to the Goldfields Railway, and the Waikino-Paeroa section, including the gorge, going to the Department of Lands and Survey.

A decade of planning, work and raising donations went into replacing the road of iron in the gorge with a 2m wide trail called the Historic Karangahake Gorge Walkway.

It was opened in stages from 1985, using local labour and subsidised temporary work scheme workers under rangers' supervision.

A 1km long railway tunnel was cleaned and fitted with electric lights to give an eerie short loop option for the riverside stroll from Karangahake Hall to Waikino Station.

Bridges were restored or replaced for foot traffic.

Signage and displays and uncovered ruins commemorated the gold mining days and townships where once thousands of people worked and lived.

Graeme Foster, author of Coromandel Walks, wrote enthusiastically in 1986 of the grandeur and the craggy bluffs of the gorge.

He said: "This must rank as one of the best-formed long walks in the country."

Now, walkers share it with cyclists.

Our ride guide, Stuart Guy, saw this potential three years ago when he heard Prime Minister John Key talking about expanding the cycle way network.

He started catering for cycling tourists as part of his Over the Top Adventures company in Waihi.

"It's ideal for family rides and, importantly, it has the variety to keep children interested and happy.

"And there is another point of difference; you can go 7km by train from Waihi to start the trail at Waikino."

As a trail bike enthusiast, used to bumpy bush tracks around Waihi, Stuart finds some of the parts boring because they are smoothly compacted gravel and weeds hide the river and specimen native trees.

But he makes his own excitement by cycling flat-out through the empty tunnel towards a keyhole of light.

Then he and photographer Greg Bowker, who brought his own machine in the car, ride ahead to find a few scary slippery slopes off trail.

Stuart is also excited by mines - and Mt Karangahake is riddled with them.

The Department of Conservation's elegantly worded signs put meat on the bones of the factories which crushed gold and silver from more than a million tonnes of the mountain's rock.

But without Stuart's local knowledge and his torch, we would have missed a thrilling window into how the rich reefs were pursued.

He led us on a detour up an old mine tramway which had been gouged into the mountain's side.

We crept along short mine tunnels, saw an old dewatering pump, and wheeled our bikes on a swingbridge over a deep river gorge.

In one tunnel, which has arched windows leading to a balcony poked in the sides, for views of the river, we met members of the Whangamata Ramblers looking similarly in awe of the work.

We returned to the Waikino Station and tea rooms, after three hours of gentle exercise in the countryside, giving us more energy to explore the side attractions and marvel at the works of a bygone era.

Next year, the gorge trail is extending west to Paeroa, giving a further 7km of flat riding.

IF YOU GOThe trail: The Karangahake Gorge section of Nga Haerenga (NZ Cycle Trail) is 7km long and runs between Waikino Station (7km from Waihi) and Karangahake (6km from Paeroa). A former 1km rail tunnel, linked by bridges across the highway and river, gives a shorter loop from Karangahake car park or lets cyclists take the tunnel one way and return via the river track. Access is off SH2 from carparks at Waikino Station and at Karangahake (across the Ohinemuri River at the Crown Battery site). The trail is Grade 1 rated, suitable for families and all ages.